Magnets on Aluminium and Brass

[pete]

I was torn between Materials, Mechanical and Electrical Engineering but could not think which would be the best home for this question so I’ve put it here for now.

I tore open an old exorcise bike that used a big fly wheel with magnets on a lever and a strip of copper around the rim. I could not find how thick the strip was but it was a big wheel so I was a bit surprised to see the copper, surely aluminium or bronze would do the trick at a fraction of the cost.

After thinking about it for a while I remembered Lenz’s effect is based on the generation of electrical currents so I’m guessing that the more conductive the metal the more magnetic properties it should show.

If I give Copper 100% conductivity, aluminium 61% and brass 28%. Would that then mean Copper will show three times more magnetic resistance in the same application as Brass?

As a side note, increasing the thickness of the metal should also increase the effect but I’m unsure how to find the minimum thickness, if anyone knows how to estimate the thickness of the metal in relation to the power of the magnet that would also be very helpful.

I’ve been trying to research this kind of stuff about eddy currents but am not doing very well at the moment so being pointed in the direction of a good source would also be very helpful.

 

[davenn]

Hi Pete

After thinking about it for a while I remembered Lenz’s effect is based on the generation of electrical currents so I’m guessing that the more conductive the metal the more magnetic properties it should show.

not sure what you mean by "magnetic properties" ?
copper is non-magnetic

If I give Copper 100% conductivity, aluminium 61% and brass 28%. Would that then mean Copper will show three times more magnetic resistance in the same application as Brass?

again, what do you mean by "magnetic resistance" ?

As a side note, increasing the thickness of the metal should also increase the effect but I’m unsure how to find the minimum thickness, if anyone knows how to estimate the thickness of the metal in relation to the power of the magnet that would also be very helpful.

well for the 3 metals you mentioned ... copper, aluminium and brass, it's irrelevant, as none of them are magnetic

I’ve been trying to research this kind of stuff about eddy currents but am not doing very well at the moment so being pointed in the direction of a good source would also be very helpful.

now eddy currents and their effects are a whole different ball game

a good experiment for you to try is to drop a bar magnet down an aluminium tube and see what happens

I won't give you the answer, instead you try it and see if you can explain what you observe happening because of eddy currents

report back with your findingsDave

 

[Baluncore]

As a side note, increasing the thickness of the metal should also increase the effect but I’m unsure how to find the minimum thickness, if anyone knows how to estimate the thickness of the metal in relation to the power of the magnet that would also be very helpful.

Find out how much time it takes for the magnetic reversal in the copper as the magnets pass a fixed point. Convert that time to a frequency.
With that frequency, you can compute the skin depth for the copper. There is no advantage in using thicker copper.

 

[pete]

Thanks for the help guys, sorry about the delayed I’m away from the screen at the mo.
That was a bit unclear. I’m talking about the Magnetic field generated by the eddy currents in the different metals described and asking if the strength of the field is directly proportional to the conductivity of the metal.
So for the example I gave would the field in copper be three times stronger than the one generated in the aluminium?
I’m presuming it corresponds to the conductivity and I’ll get some aluminium and copper plates into test both and I’m guessing that though the copper will produce a stronger magnetic field I can't imagine it being three times stronger than the copper? But that’s just from my limited experience fiddling with these things.
Thanks for the help on the thickness question by the way.